Method and apparatus for modulating light

ABSTRACT

Embodiments relate to a method and apparatus for producing polarized light, having a modulator crystal, where the modulator crystal incorporates a birefringent electro-optic material. The modulator crystal has an optic axis, a first polarization axis, and a second polarization axis, where the first polarization axis and second polarization axis are each perpendicular to the optic axis and perpendicular to each other. The apparatus can also include an electrode pair, where application of an electric field modulates light passing through the modulator crystal that is polarized along the first polarization axis. Embodiments pertain to a method and apparatus for modulating light. The apparatus incorporates a modulator crystal having an electro-optic material. The device also has at least two electrode pairs, where each electrode pair modulates light passing through the modulator crystal that has a direction of travel that has a component parallel to the optic axis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/996,695, filed Dec. 7, 2010, which is a national stageapplication of International Patent Application No. PCT/US2009/050417,filed Jul. 13, 2009, which claims the benefit of U.S. ProvisionalApplication Ser. No. 61/080,129, filed Jul. 11, 2008, the disclosures ofwhich are hereby incorporated by reference in their entireties,including any figures, tables, or drawings.

The subject invention was made with government support under a researchproject supported by the National Science Foundation, Contract NumbersPHY-0555453 and PHY-0244902. The government has certain rights in thismatter.

BACKGROUND OF INVENTION

Phase and amplitude modulation of light is usually generated by theelectro-optic effect, where the index of refraction of a dielectricmaterial is changed by applying a variable external electric field.Typically, a sinusoidal electric field with a fixed frequency is appliedacross a birefringent electro-optic crystal. This effect can be used togenerate phase modulation, amplitude modulation, and/or polarizationrotation/variation depending on the polarization state of the incidentlight and the orientation of the crystal. Current designs typically useone pair of electrodes per modulator material block to apply theelectric field, as shown in FIG. 1. The modulator material is commonlymade of a transparent, crystalline medium with electro-optic properties,which can be referred to as a modulator crystal. When multiple, e.g., 3,modulation frequencies are required, multiple, e.g., 3, modulators arecurrently used to provide the multiple frequencies.

Phase modulation of light is often generated by utilization of theelectro-optic effect, where the index of refraction of a dielectricmaterial is changed by applying a variable external electric field,typically a sinusoidal voltage at a fixed frequency, applied across thecrystal, perpendicular to the direction of travel of the light. Theelectro-optic effect can also be used to generate amplitude modulation,and/or to rotate the polarization of the incident light, by adjustingthe polarization state of the incident light and the orientation of thecrystal. The generation of amplitude modulation, when certainorientations of the polarization state of the incident light and theorientation of the crystal occur, makes it hard to achieve pure phasemodulation without spurious, unwanted, amplitude modulation orpolarization rotation. The modulator material is usually made of atransparent, crystalline medium with electro-optic properties, which canbe referred to as a modulator crystal. Current phase modulator designsuse crystal front faces that are parallel, as shown in FIG. 1. In thedesign shown in FIG. 1, both polarizations (x and z) of the incomingbeam remain superimposed in the outgoing beam.

BRIEF SUMMARY

Embodiments of the invention can utilize multiple pairs of electrodessequentially positioned on a single modulator crystal, as shown in FIG.2. Embodiments utilizing multiple electrode pairs on a single modulatorcrystal can reduce the number of optical surfaces per modulationfrequency when multiple modulation frequencies need to be applied to alight field. In applications where low optical losses are important,keeping the number of optical surfaces low can reduce the overalllosses.

Embodiments of the invention can use a wedged modulator crystal madefrom a birefringent electro-optic material that acts as a polarizer toseparate the incident light into light with the polarizations collinearwith the x- or z-axis of the modulator crystal, as shown in FIG. 3.Further embodiments can utilize a modulator crystal with one or theother front faces having a normal that makes an angle with respect tothe optic axis (e.g., y-axis) of the crystal, such that the opposingfront faces are not parallel. A birefringent material with differentvalues for the index of refraction for the x- and z-axis of the crystalallows the separation of the incident light into the polarizationscollinear with the x- or z-axis of the modulator crystal. Materials thatcan be used with embodiments of the subject invention include, but arenot limited to, rubidium titanyl phosphate (RbTiOPO₄ or RTP), rubidiumtitanyl arsenate (RbTiOAsO₄ or RTA), and lithium niobate (LiNbO₃).

Further embodiments can incorporate at least two electrode pairs with abirefringent electro-optic modulator crystal having at least one facefront having a normal that makes an angle with respect to the optic axis(e.g., y-axis) of the crystal, such that the face fronts are notparallel. A specific embodiment can merge the features shown in FIGS. 2and 3.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a conventional modulator.

FIG. 2 shows a three electrode pair modulator in accordance with anembodiment of the subject invention.

FIG. 3 shows a wedged modulator crystal separating the polarizations inaccordance with an embodiment of the subject invention.

DETAILED DISCLOSURE

Embodiments pertain to a method and apparatus for modulating light. Theapparatus incorporates a modulator crystal having a electro-opticmaterial. The modulator crystal has an optic axis, a first polarizationaxis, and a second polarization axis, where the first polarization axisand the second polarization axis are each perpendicular to the opticaxis and perpendicular to each other. The device also has at least twoelectrode pairs, where each electrode pair is positioned such that whena voltage is applied across the electrode pair an electric field iscreated through a portion of the modulation crystal. The electric fieldhas at least a component perpendicular to the optic axis that modulatesthe light. The application of the electric field modulates light passingthrough the modulator crystal that has a direction of travel that has acomponent parallel to the optic axis. Preferably, the light travelsalong the optic axis. In specific embodiments, the electro-opticmaterial is LiNbO₃, RbTiOAsO₄, or RbTiOPO₄.

In specific embodiments, the electric field created by at least one ofthe at least two electrode pairs is a time-varying electric field havinga fixed frequency. The electric field created by at least one of the atleast two electrode pairs can be a sinusoidal electric field. Inspecific embodiments, each of the electric fields from the at least twoelectrode pairs are at a different frequency. The electric field canphase modulate the light passing through the modulator crystal,amplitude modulate the light passing through the modulator crystal, varya polarization of the light passing through the modulator crystal,and/or rotate a polarization of the light passing through the modulatorcrystal.

In specific embodiments, at least one of the electric fields from the atleast two electrode pairs has a component parallel to the firstpolarization axis. In specific embodiments, the modulator crystal canhave a front surface for receiving light that is to pass through themodulator crystal, where the front surface lies in a plane that isperpendicular to the optic axis. The at least two electrode pairs canhave 3 to 4 electrode pairs, 3 to 16 electrode pairs, 3 to 32 electrodepairs, and/or 3 to 64 electrode pairs. In specific embodiments, two ormore of the at least two electrode pairs can share a common groundelectrode and/or the electrode can use interdigitated electrode portionsas known in the art. The electric field alters the wavelength of thelight passing through the modulator crystal for light having apolarization component in the direction of the electric field.

In specific embodiments, electrodes of the at least two electrode pairsare positioned in parallel planes that are parallel to the optic axis.

Embodiments of the invention can utilize multiple pairs of electrodessequentially positioned on a single modulator crystal, as shown in FIG.2. Embodiments utilizing multiple electrode pairs on a single modulatorcrystal can reduce the number of optical surfaces per modulationfrequency when multiple modulation frequencies need to be applied to alight field. In applications where low optical losses are important,keeping the number of optical surfaces low can reduce the overalllosses.

The use of multiple electrode pairs on a single modulator crystal allowsthe application of multiple modulations using only one crystal, thusreducing surface reflections and, therefore, optical losses, as comparedto the use of multiple separate modulators for each modulation applied.The use of multiple electrode pairs on a single modulator crystal canalso reduce the number of required modulator crystals, crystal housings,and optical mounts for the housings. As these components can beexpensive, overall cost required to generate multiple modulationfrequencies can be reduced.

Specific embodiment relate to a method and apparatus for producingpolarized light, having a modulator crystal, where the modulator crystalincorporates a birefringent electro-optic material. The modulatorcrystal has an optic axis, a first polarization axis, and a secondpolarization axis, where the first polarization axis and secondpolarization axis are each perpendicular to the optic axis andperpendicular to each other. Light can enter a first end of themodulator crystal at a first surface and exits a second end of themodulator crystal at a second surface, where the first surface of thefirst end lies in a first plane and the second surface of the second endlies in a second plane, where the first plane and the second place arenot parallel. The apparatus can also include an electrode pair, wherethe electrode pair is positioned such that when a voltage is appliedacross the electrode pair an electric field is created through a portionof the modulation crystal. The electric field can have at least acomponent perpendicular to the optic axis, where application of theelectric field creates a difference in a first index of refraction forlight polarized along the first polarization axis so as to modulatelight passing through the modulator crystal that is polarized along thefirst polarization axis. When light is incident on the first end, passesthrough the modulator crystal, and exits the second end, the light issplit into a first beam that is polarized along the first axis ofpolarization and a second beam that is polarized along the second axisof polarization as the light exits the second end, where the first beamand the second beam diverge from each other as the light exits thesecond end. By diverging, the beams can be separated after leaving thesecond end of the device. In specific embodiments, after traveling 10meters from the second end the first beam and second are sufficientlyseparated from each other that amplitude modulation from thesuperposition of the first beam and the second beam is strongly reducedor eliminated. In specific embodiments, after traveling 1 meter from thesecond end the first beam and second are sufficiently separated fromeach other that amplitude modulation from the superposition of the firstbeam and the second beam is strongly reduced or eliminated. In specificembodiments, the birefringent electro-optic material can be RbTiOAsO₄,LiNbO₃, or RbTiOPO₄.

In specific embodiments, the first plane makes a first angle withrespect to a normal plane that is normal to the optic axis. In specificembodiments, the second plane makes a second angle with respect to anormal plane that is normal to the optic axis. In specific embodiments,the first plane makes a first angle with respect to a normal plane thatis nominal to the optic axis and the second plane makes a second anglewith respect to the normal plane. The second angle can have a secondmagnitude that is the same as a first magnitude of the first angle ordifferent. The first angle can have an opposite orientation to the opticaxis as the second angle or the same. The first plane can makes a firstadditional angle with an additional normal plane that is normal to thefirst polarization axis, wherein the first plane is parallel to thesecond polarization axis. The first polarization axis is preferably theaxis of light polarization having the largest index of refraction. Thesecond plane can also make a second additional angle with an additionalnormal plane that is normal to the first polarization axis, where thesecond plane is parallel to the second polarization axis. In specificembodiments, the first plane makes a first additional angle with anadditional normal plane that is normal to the first polarization axis,the first plane is parallel to the second polarization axis, the secondplane makes a second additional angle with the additional normal plane,and the second plane is parallel to the second polarization axis. Inspecific embodiments, the first angle has a first magnitude of at least1° and/or the second angle has a second magnitude of at least 1°.

A filter can be positioned to block the first beam after the first beamexits the second end and allows the second beam to pass. Othertechniques can also be used to isolate the second beam.

In specific embodiments, the electric field created by the electrodepair is a sinusoidal electric field. The application of the electricfield can phase modulate the light passing through the modulatorcrystal, amplitude modulate the light passing through the modulatorcrystal, vary a polarization of the light passing through the modulatorcrystal, and/or rotate a polarization of the light passing through themodulator crystal.

Embodiments of the invention can use a wedged modulator crystal madefrom a birefringent electro-optic material that acts as a polarizer toseparate the incident light into light with the polarizations collinearwith the x- or z-axis of the modulator crystal, as shown in FIG. 3.Further embodiments can utilize a modulator crystal with one or theother front faces having a normal that makes an angle with respect tothe optic axis (e.g., y-axis) of the crystal, such that the opposingfront faces are not parallel. A birefringent material with differentvalues for the index of refraction for the x- and z-axis of the crystalallows the separation of the incident light into the polarizationscollinear with the x- or z-axis of the modulator crystal. Materials thatcan be used with embodiments of the subject invention include, but arenot limited to, rubidium titanyl phosphate (RbTiOPO₄ or RTP), rubidiumtitanyl arsenate (RbTiOAsO₄ or RTA), and lithium niobate (LiNbO₃).

RTP is used as the electro-optic crystal in specific embodiments of theinvention. RTP has an excellent electro-optic coefficient for lightpolarized along the z-axis. The index of refraction for the z-axis isn_(z)=1.9. For the x-axis, the index of refraction is n_(x)=1.8.

The two refractive indices lead to different angles of refraction forlight incident on the surface at an angle. In the case of the wedgedcrystal shown in FIG. 3, the incident light is bent towards the normalentering the crystal and bent away from the normal leaving. Bothpolarizations get a net deflection angle but, because the refractiveindices are different, the prism refraction angles are different.Therefore, the two polarizations leave the crystal at different angles.In a specific embodiment, the unwanted x-polarized light is blocked byan aperture behind the modulator.

The device shown in FIG. 3 uses a wedge angle of 2.85 degrees at eachfront face. Light polarized along the z-axis is deflected by 5.2 degreeswhereas the undesirable x-axis polarized light is deflected by 4.7degrees. Were the two polarizations to proceed together to the detector,they would combine to give modulation of the polarization state, and,hence to give amplitude modulation. The spatial separation allows theremoval of one of the two polarized light fields in order to avoid therecombination of the light fields and, therefore, any change ofpolarization state. The z-axis polarized light can be purely phasemodulated and stay as a pure linear polarization. Accordingly, the useof birefringent electro-optic crystals having non-parallel front facescan be used to generate pure phase modulated light.

Embodiments of the subject invention can utilize birefringentelectro-optic crystal material, such as rubidium titanyl phosphate(RbTiOPO₄ or RTP), rubidium titanyl arsenate (RbTiOAsO₄ or RTA), andlithium niobate (LiNbO₃). In a specific embodiment, RTP was used as themodulator material.

For a specific embodiment of a modulator using RTP, the crystaldimension is 4×4×40 mm, with the long dimension being aligned to they-axis of the crystal. The dimensions where chosen to be large enough toaccommodate a high power laser beam while keeping the half-wave voltageat a reasonably low level. To avoid the unwanted generation of amplitudemodulation by polarization modulation because of imperfect alignment ofthe incident light and also, to remove etalon interference effects, thefaces of the RTP crystal were wedged by 2.85 degrees against the z-axisof the crystal. Other angles can be implemented. Preferably, an angle ofat least 1° is used. Preferably, an angle is chosen so that there is nototal internal reflection. The birefringence of the RTP materialseparates the different polarizations by approximately 0.5 degrees,allowing removing of one of the polarized light fields in order to avoidthe rotation of the polarization that leads to amplitude modulation.Preferably, the polarized light field to be retained travels parallel tothe y-axis of the crystal, which can be achieved by selection of theangle of incidence of the incoming light beam. The crystal faces can beanti-reflection (AR) coated to achieve less than 0.1% remainingreflectivity.

As a way to reduce the optical losses due to remaining surfacereflections, the number of modulator crystals can be reduced from threeto one with three separate pairs of electrodes creating the threeelectric fields needed for modulation. This allows the application ofthree different modulation frequencies. The length of the centerelectrode can be increased to achieve a stronger modulation depth.

The three electrode pairs can be separately driven with a resonantcircuit that is designed to have an input impedance of 50 Ω. In aspecific embodiment, the modulation frequencies of the modulators are33.0 MHz, 24.5 MHz, and 61.2 MHz and with a drive power of 24 dBm foreach electrode pair the corresponding modulation depths are 0.14, 0.37and 0.14 generated. The modulations can still be applied sequentially asin the case for discrete modulators.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

1. An apparatus for modulating light, comprising: a modulator crystal,wherein the modulator crystal comprises an electro-optic material,wherein the modulator crystal has an optic axis, a first polarizationaxis, and a second polarization axis, wherein the first polarizationaxis and the second polarization axis are each perpendicular to theoptic axis and perpendicular to each other, at least two electrodepairs, wherein each electrode pair is positioned such that when avoltage is applied across the electrode pair an electric field iscreated through a portion of the modulator crystal, wherein the electricfield has at least a component perpendicular to the optic axis, whereinapplication of the electric field modulates light passing through themodulator crystal that has a direction of travel that has a componentparallel to the optic axis.
 2. The apparatus according to claim 1,wherein the electro-optic material is selected from the group consistingof: LiNbO₃ and RbTiOAsO₄.
 3. The apparatus according to claim 1, whereinthe electro-optic material is RbTiOPO₄.
 4. The apparatus according toclaim 1, wherein the electric field created by at least one of the atleast two electrode pairs is a time-varying electric field having afixed frequency.
 5. The apparatus according to claim 1, wherein theelectric field created by at least one of the at least two electrodepairs is a sinusoidal electric field.
 6. The apparatus according toclaim 1, wherein each of the electric fields from the at least twoelectrode pairs is at a different frequency.
 7. The apparatus accordingto claim 1, wherein the application of the electric field phasemodulates the light passing through the modulator crystal.
 8. Theapparatus according to claim 1, wherein the application of the electricfield amplitude modulates the light passing through the modulatorcrystal.
 9. The apparatus according to claim 1, wherein the applicationof the electric field varies a polarization of the light passing throughthe modulator crystal.
 10. The apparatus according to claim 1, whereinthe application of the electric field rotates a polarization of thelight passing through the modulator crystal.
 11. The apparatus accordingto claim 1, wherein at least one of the electric fields from the atleast two electrode pairs comprises a component parallel to the firstpolarization axis.
 12. The apparatus according to claim 1, wherein themodulator crystal has a front surface for receiving light that is topass through the modulator crystal, wherein the front surface lies in aplane that is perpendicular to the optic axis.
 13. The apparatusaccording to claim 1, wherein the at least two electrode pairs comprises3 to 4 electrode pairs.
 14. The apparatus according to claim 1, whereinthe at least two electrode pairs comprises 3 to 16 electrode pairs. 15.The apparatus according to claim 1, wherein the at least two electrodepairs comprises 3 to 32 electrode pairs.
 16. The apparatus according toclaim 1, wherein the at least two electrode pairs comprises 3 to 64electrode pairs.
 17. The apparatus according to claim 1, wherein two ormore of the at least two electrode pairs share a common groundelectrode.
 18. The apparatus according to claim 1, wherein the electricfield alters the wavelength of the light passing through the modulatorcrystal for light having a polarization component in the direction ofthe electric field.
 19. The apparatus according to claim 1, whereinelectrodes of the at least two electrode pairs are positioned inparallel planes that are parallel to the optic axis.
 20. A method formodulating light, comprising: providing a modulator crystal, wherein themodulator crystal comprises an electro-optic material, wherein themodulator crystal has an optic axis, a first polarization axis, and asecond polarization axis, wherein the first polarization axis and thesecond polarization axis are each perpendicular to the optic axis andperpendicular to each other, providing at least two electrode pairs,wherein each electrode pair is positioned such that when a voltage isapplied across the electrode pair an electric field is created through aportion of the modulator crystal, wherein the electric field has atleast a component perpendicular to the optic axis, wherein applicationof the electric field modulates light passing through the modulatorcrystal that has a direction of travel that has a component parallel tothe optic axis; incidenting a light beam on a first end of the modulatorcrystal wherein the light beam enters the modulator crystal and travelsin a direction having a component parallel to the optic axis; applying afirst voltage across a first pair of the at least two pairs ofelectrodes so as to modulate the light beam; applying a second voltageacross a second pair of the at least two pairs of electrodes so as tomodulate the light beam.
 21. The method according to claim 20, whereinthe electro-optic material is selected from the group consisting of: andRbTiOAsO₄ and LiNbO₃.
 22. The method according to claim 20, wherein theelectro-optic material is RbTiOPO₄.
 23. The method according to claim20, wherein the electric field created by applying the first voltage tothe first pair of the at least two electrode pairs is a time-varyingelectric field having a fixed frequency.
 24. The method according toclaim 20, wherein the electric field created by applying the firstvoltage to the first pair of the at least two electrode pairs is asinusoidal electric field.
 25. The method according to claim 20, whereinthe first voltage and the second voltage are at different frequencies.26. The method according to claim 20, wherein applying the first voltagemodulates the light passing through the modulator crystal.
 27. Themethod according to claim 20, wherein the application of the electricfield amplitude modulates the light passing through the modulatorcrystal.
 28. The method according to claim 20, wherein the applicationof the electric field varies a polarization of the light passing throughthe modulator crystal.
 29. The method according to claim 20, wherein theapplication of the electric field rotates a polarization of the lightpassing through the modulator crystal.
 30. The method according to claim20, wherein at least one of the electric fields from the at least twoelectrode pairs comprises a component parallel to the first polarizationaxis.
 31. The method according to claim 40, wherein the modulatorcrystal has a front surface for receiving light that is to pass throughthe modulator crystal, wherein the front surface lies in a plane that isperpendicular to the optic axis.
 32. The method according to claim 20,wherein the at least two electrode pairs comprises 3 to 4 electrodes.33. The method according to claim 20, wherein the at least two electrodepairs comprises 3 to 8 electrodes.
 34. The method according to claim 20,wherein the at least two electrode pairs comprises 3 to 16 electrodes.35. The method according to claim 20, wherein the at least two electrodepairs comprises 3 to 32 electrodes.
 36. The method according to claim20, wherein the at least two electrode pairs comprises 3 to 64electrodes.
 37. The method according to claim 20, wherein two or more ofthe at least two electrode pairs share a common ground electrode. 38.The method according to claim 20, wherein the electric field alters thewavelength of the light passing through the modulator crystal for lighthaving a polarization component in the direction of the electric field.39. The method according to claim 20, wherein electrodes of the at leasttwo electrode pairs are positioned in parallel planes that are parallelto the optic axis.